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Question: Why don't humans have gills?
Answer: From this question, it sounds like you may want a set of gills to do some long underwater swimming this summer. Growing up in Florida, I too would have benefited from a pair of gills. However, before you get too anxious and try your lungs out as gills, let us talk a little about the difference between them. Both humans and fish need oxygen delivered to their cells to survive. We call the process that takes oxygen from an environment and delivers it to the cells, respiration. Humans, as you know, live on land where they are surrounded by air. Fish live in rivers, lakes, and oceans where they are surrounded by water. Because air and water are quite different, in fact, one is a gas and the other a liquid, one would expect differences in the way respiration is carried out. The main organs that carry out respiration are the lungs for the human and the gill for the fish. To understand why there are two different organs to do the same thing, we need to look at two key differences between these environments: 1) the ease in which you can move through the substance. 2) The amount of oxygen in the environment. Air is composed of different gases and therefore you can move through air easily as the molecules are not close together on the average. Try moving your hand quickly through the air in front of you to see how easily it moves. Now try moving your hand through water in a tub or pool. It takes much more effort. Water is about 60 times more viscous or thicker than air; therefore, more muscle power is needed to move it. This is due to the fact that the molecules are much closer together. So, you can see that lungs and gills must work differently in the way they move their environment through them. The oxygen in water is also a gas but is dissolved in liquid water. Oxygen is not very soluble in water. Under normal circumstances, about 12 parts of oxygen can dissolve in a million parts of water (12 mg/liter). This fact alone is enough reason why water would not be a good environment for humans. Humans are warm-blooded which means the body converts some of its food into heat to keep the body warm. In order to convert food to heat, a lot of oxygen is needed, more than there is available in water. In fact, the air we breathe has an oxygen content of about 20%. Fish, however, are cold-blooded, keeping their bodies warm from their environment directly; therefore, they do not need oxygen for converting food to heat. So, you can see that the difference in oxygen content would require different organs for respiration. These are two good reasons why you have lungs and not gills. So, please hold your breath this summer before you go underwater to look at the fish.
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